1739 查看數據表（PDF） - Linear Technology

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1739

Dual 500mA, 200MHz xDSL Line Driver Amplifier

Linear Technology 

LT1739

APPLICATIO S I FOR ATIO

Layout and Passive Components

With a gain bandwidth product of 200MHz the LT1739

requires attention to detail in order to extract maximum

performance. Use a ground plane, short lead lengths and

a combination of RF-quality supply bypass capacitors (i.e.,

0.1µF). As the primary applications have high drive cur-

rent, use low ESR supply bypass capacitors (1µF to 10µF).

The parallel combination of the feedback resistor and gain

setting resistor on the inverting input can combine with the

input capacitance to form a pole that can cause frequency

peaking. In general, use feedback resistors of 1k or less.

Compensation

The LT1739 is stable in a gain 10 or higher for any supply

and resistive load. It is easily compensated for lower gains

with a single resistor or a resistor plus a capacitor.

Figure␣ 9 shows that for inverting gains, a resistor from the

inverting node to AC ground guarantees stability if the

parallel combination of RC and RG is less than or equal to

RF/9. For lowest distortion and DC output offset, a series

capacitor, CC, can be used to reduce the noise gain at

lower frequencies. The break frequency produced by RC

and CC should be less than 5MHz to minimize peaking.

Figure 10 shows compensation in the noninverting con-

figuration. The RC, CC network acts similarly to the invert-

ing case. The input impedance is not reduced because the

network is bootstrapped. This network can also be placed

between the inverting input and an AC ground.

Another compensation scheme for noninverting circuits is

shown in Figure 11. The circuit is unity gain at low

frequency and a gain of 1 + RF/RG at high frequency. The

DC output offset is reduced by a factor of ten. The

techniques of Figures 10 and 11 can be combined as

shown in Figure 12. The gain is unity at low frequencies,

1 + RF/RG at mid-band and for stability, a gain of 10 or

greater at high frequencies.

RG

VI

RC

CC

(OPTIONAL)

RF

–

+

VO

VI

=

–RF

RG

VO

(RC || RG) ≤ RF/9

1 < 5MHz

2πRCCC

1739 F09

Figure 9. Compensation for Inverting Gains

VI

RC

CC

(OPTIONAL)

+

–

RF

VO

VI

=1+

RF

RG

VO (RC || RG) ≤ RF/9

1

2πRCCC

<

5MHz

RG

1739 F10

Figure 10. Compensation for Noninverting Gains

+

Vi

–

RF

RG

CC

VO

VI

=

1

(LOW

FREQUENCIES)

VO

=

1

+

RF

RG

(HIGH

FREQUENCIES)

RG ≤ RF/9

1

2πRGCC

<

5MHz

1739 F11

Figure 11. Alternate Noninverting Compensation

VI

RC

CC

RG

CBIG

+

–

RF

VO

VO

VI

= 1 AT LOW FREQUENCIES

=

1

+

RF

RG

AT

MEDIUM

FREQUENCIES

= 1 + RF AT HIGH FREQUENCIES

(RC || RG)

1739 F12

Figure 12. Combination Compensation

12

1739fas, sn1739

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